Pony Carbs 'spreadbore'

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Apr 12, 2009 | 02:42 PM

F-250 restorer

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Pony Carbs 'spreadbore'

Hey guys:

I wanted to run something by you which has been bothering me. I am about to send out an autolite 4100 to pony carbs to have them make one of their 'spreadbores'. They take the 1.12 venturi, (600 cfm) and install a shim or sleeve in the primaries, reducing the size to 1.00", and the secondaries become 1.19. All the sites say it works great, great milage, etc.

What bothers me is this: by narrowing the primaries, they are altering the velocity of the air flow, making it faster, especially going into the dp manifold I have, with the small primary runners. It seems that would also change the vacuum operated secondaries function, making them open sooner than normal, thereby cancelling out economy, the purpose of going with the 'spreadbore'. It also seems that the air flow, divided into the small runners of the offy dp intake, would be flowing so quickly, that it might lunge at take off. A bit of feed back here would be really welcome.

When I speak with Pony Carbs, they seem to tell me what I want to hear. Their set up is not cheap, and I'm trying to do the research before investing the $.

I need to decide whether to go with the spreadbore, or put on the smaller 1.o8 venturi (480 cfm) I have with the mustang numbers, designed for the 289.

I have both carbs, so I'm definetely staying with the autolite 4100. Thanks.

Apr 12, 2009 | 03:57 PM

Harte3

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Interesting question. I don't know the answer. But a lack of velocity is usually the problem causing a weak signal in the carb for the proper fuel flow.

Apr 12, 2009 | 05:05 PM

91Bronc300

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Is the vacuum setpoint for the secondaries not adjustable on those carbs? Shimming down the primaries would cause the vacuum signal to increase for a given airflow amount, causing your secondaries to open up at a smaller airflow than stock, but that would only be an issue if the secondary opening point is not adjustable. But being that those carbs have annular boosters why do you need to shim down the primaries? You should still have plenty of vacuum signal to get a very nice air/fuel mixture. That's why those carbs are so sought after in the first place. I don't have any input about your question about lunging because I couldn't even guess there, but I think you would be better served to use the carb as it is. Especially given the expense of chaning those venturi diameters

Apr 12, 2009 | 07:18 PM

Harte3

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From: Spokane, WA

Tuning an Autolite 4100 Carburetor - Google Search

There are some articles on the 4100. I read one of them and it seemed like everything but secondary adjustment was covered. I'm just guessing but it might have an Qjet type, air flap system and should be adjustable.

Apr 14, 2009 | 01:47 AM

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pony carbs 'spreadbore'

Quote:

Originally Posted by 91Bronc300

Especially given the expense of chaning those venturi diameters

The reason to change the venturi is that the 4100 with the 1.12 venturi is a 600cfm carb, and too big for that engine.

EXPENSE: The shims to narrow the venturi are only $40. But what concerns me is that my engine has over size 1.94 intake valves, and is going to be pumping more air than stock, thereby sucking MORE mix through the carb. Thats much greater velocity for a 1.00" venturi, and thereby more vacuum.

I think I'll just stay with the stock 1.08 venturi (480cfm), and set the float level for a 302 sb, and see where that gets me.

Thanks,